Oxide supports provide an ideal environment for metal ions because such surfaces are thermally and chemically robust and prevent metal ions from undergoing bimolecular degradation pathways. Some useful catalytic reactions in which metal-oxide (or silicon oxide) surfaces play an important role are oxidative coupling of alkanes, alkane metathesis, transfer dehydrogenation and hydrogenation, hydrodenitrogenation, hydrodeoxygenation and hydrodesulfurization. The latter three transformations are indispensable in the petrochemical industry since they are needed steps in the purification of crude oil from nitrogen-, oxygen- and sulfur-based impurities. The first three reactions are now becoming very important because of energy concerns and present prices of crude oil, but are unfortunately under-developed.
Another important role of metal oxide surfaces is in the Haber-Bosch process which consumes about 1% of the total worlds energy supply for the conversion of N2 and H2 into NH3, the building block for most nitrogen-containing reagents used in chemistry or as fertilizers (e.g. nitrates, nitrites, nitriles, and amines). Unfortunately, understanding the role of the metal ion in these important processes has been difficult because of the heterogeneous nature of the catalyst and the ill-defined nature of the active site. Therefore, the development of homogeneous (i.e., in the same phase of the reactants) catalysts for reactions traditionally catalyzed by metal-oxide surfaces would provide insight into the mechanism of such reactions, thereby enabling further improvements to the catalysts themselves. Such improvements would be particularly advantageous for the oxidative coupling of alkanes, alkanes metathesis, and transfer dehydrogenation and hydrogenation, since such transformations are increasing in their importance to the petrochemical industry but are still poorly understood from a mechanistic and catalytic perspective.
Past work in developing aryloxide chemistry as a model for oxide surfaces has suffered from three major drawbacks: (i) access to thermally robust and sterically encumbered aryl oxides can be difficult to access via coupling routes [3] and difficult chemical synthesis steps [4]; (ii) commercially available bulky aryl oxides often suffer from degradation pathways such as cyclometallation, oligomerization or demetallation, [4-17] and (iii) known platforms for modeling metal oxides saturate the metal center or render it exposed. [18-26] Placing more chemically robust groups has been found to improve thermal stability, but only temporarily. [19,27]